Fixing extraneous semicolon in jsgen output

jsgen was producing javascript objects like this
```
{, name:"foo"}
```
causing syntax errors in javascript interpretors.

compiler/jsgen.nim

@@ -1431,7 +1431,7 @@ proc genObjConstr(p: PProc, n: PNode, r: var TCompRes) =
   r.res = toRope("{")
   r.kind = resExpr
   for i in countup(1, sonsLen(n) - 1):
-    if i > 0: app(r.res, ", ")
+    if i > 1: app(r.res, ", ")
     var it = n.sons[i]
     internalAssert it.kind == nkExprColonExpr
     gen(p, it.sons[1], a)